Population density ofMicrorhopala vittata(F.) (Coleoptera: Chrysomelidae), a leaf-miner of goldenrod (Solidago), is low in most old fields, but populations occasionally reach densities at which they severely damage their host plant. A survey of 21 old fields revealed that the beetle was present at all sites. To determine whether development was retarded or mortality was elevated in sites with fewer beetles, growth and mortality were experimentally assessed in three low-density sites and three high-density sites. A major source of mortality was early leaf senescence in sites with taller plants, and in these sites larvae grew more slowly. This form of mortality was not present in all three low-density sites, however, and thus was not sufficient to explain differences in beetle density. Only eulophid parasitism of larvae was more prevalent in the low-density sites. Unexplained mortality of first instars was greater in high-density sites. However, overall egg-to-pupa mortality was not correlated with initial beetle density. The combined effect of several mortality agents may be responsible for maintainingM. vittatapopulations at low densities most of the time. A release from these agents that would allow populations to irrupt may be short-lived and thus difficult to detect.